1. Field of the Invention
The present invention relates to a magnesium-containing metal granulate for the storage of hydrogen in magnesium or in an alloy-containing magnesium, which includes granulate particles having a catalyst applied on the surfaces thereof.
In contrast to other metals or metal alloys, especially such metal alloys which contain titanium or lanthanum, magnesium is preferred for the storage of hydrogen not only because of its lower material costs, but above all, because of its lower specific weight as a storage material. However, the hydrogenation EQU Mg+H.sub.2 .fwdarw.MgH.sub.2
is, in general, more difficult to achieve with magnesium, inasmuch as the surface of the magnesium will rapidly oxidize in air so as to form stable MgO and/or Mg(OH).sub.2 surface layers. These layers inhibit the dissociation of hydrogen molecules, as well as the adsorption of produced hydrogen atoms and their conductance from the surface of the granulate particles into the magnesium storage mass. Efforts are being made to ameliorate through catalysts the disadvantages which are hereby encountered during the storage of hydrogen in magnesium.
2. Discussion of the Prior Art
It is known that the storage quality of magnesium and magnesium alloys can be enhanced in that magnesium, which forms stable oxides (and stable hydroxides), is alloyed with a metal phase which forms unstable oxides (and unstable hydroxides). The comments made with respect to the oxides are analogous for stable and unstable hydroxides under normal hydrogenation conditions, for such an alloy the partial pressure of oxygen is only sufficient for the formation of stable oxides. The formers for unstable oxides are reduced during the hydrogenation and remain present as metals. For example, such an alloy is Mg.sub.2 Ni, in which the Ni forms unstable oxides. In this alloy, thermodynamic examinations indicated that the surface reaction Mg.sub.2 Ni+O.sub.2 .fwdarw.2MgO+Ni extended over nickel metal inclusions which catalyze the dissociation-adsorption reaction. Reference may be had to A. Seiler et al., Journal of Less-Common Metals 73, 1980, pages 193 et seq.
A further possibility for the catalysis of the dissociation-adsorption reaction on the surface of magnesium lies also in the formation of a two-phase alloy, wherein the one phase is a hydride former, and the other phase is a catalyst. Thus, it is known to employ galvanically-nickeled magnesium as a hydrogen storage, referring to F.G. Eisenberg et al. Journal of Less-Common Metals 74, 1980, pages 323 et seq. However, there were encountered problems during the adhesion and the distribution of the nickel over the magnesium surface.
In order to obtain an extremely dense and good adherent catalyst phase under the formation alone of equilibrium phases, it is also known to that for the storage of hydrogen there can be employed an eutectic mixture of magnesium as a hydride-forming phase in conjunction with magnesium copper (Mg.sub.2 Cu), referring to J. Genossar et al., Zeitschrift fur Physikalische Chemie Neue Folge 116, 1979, pages 215 et seq. The storage capacity per volume of work material which is achieved through this magnesium-containing granulate does not, however, meet any high demands because of the quantity of magnesium copper which is required for the eutectic mixture.